1. Field of the Invention
The present invention relates to a method of recovering errors in a hard disk drive (HDD), and more particularly, to an off-track retry method for recovering off-track errors.
2. Description of the Related Art
Data can be recorded on each sector of a hard disk drive (HDD). If an error occurs during recording/reading of data on/from a desired sector of a data storage medium it is possible to try to record/read the data again to the disk head on the desired track of the data storage medium when the desired track comes back again while the data storage medium is rotating, which is called a retry.
There are different retry methods available in a hard disk drive, capable of recovering errors triggered by different causes. One of the retry methods is an off-track retry method.
An off-track error indicates an error occurring when data is recorded off the centerline of a desired track rather than on the centerline of the desired track. Therefore the data cannot be read via a normal tracking method.
When recording data on a hard disk drive, data may be recorded off-track due to repeatable run-out or non-repeatable run-out components inside and outside of the hard disk drive. It is impossible to read-the data from the hard disk drive by a normal tracking method in which a head is placed over the centerline of a track where the data is expected to be.
FIG. 1 is a flowchart of a conventional off-track retry method. Referring to FIG. 1, a disk read command to read data from a specific address of a hard disk drive is applied from a host in operation s102. When data has been recorded off-track at a corresponding address, a read error occurs in operation s104 because it is impossible to read the data via a typical error recovery method.
There may be many reasons for a read error,other than an off-track error. The hard disk drive performs various error recovery methods as part of a retry routine. An off-track retry method is a type of error recovery method.
When a read error occurs, the hard disk drive performs predetermined retry methods including an off-track retry method in order to correct the read error in operation s106.
According to the conventional off-track retry method, a head is forced to be offset from either side of the centerline of a desired track by a predetermined distance in order to determine whether data is read correctly, i.e., whether a read error is recovered. Generally, the head is moved toward either side of the desired track from the centerline of the desired track by as much as ±15%. The degree to which the data is recorded off the centerline of a desired track where the data was supposed to be recorded is represented by a percentage, and ‘+’ and ‘−’ indicating the off-track directions.
Then, it is determined whether the data has been read correctly in operation s108. If the data has been read correctly, the next command is performed in operation s110. If the data has not been read correctly, a read failure is announced in operation s112.
According to the conventional off-track error recovery method shown in FIG. 1, whether the data has been read correctly is determined based upon the result of reading data from places offset either side of the centerline of a desired track in ‘+’ and ‘−’ directions by a predetermined off-track amount.
Since a hard disk drive cannot figure out how far off-track the data is offset from the centerline of a desired track where the data was supposed to be recorded at a predetermined address, it is determined whether or not the data has been read correctly, based upon the result of reading data which is offset from either side of the desired track in ‘+’ and ‘−’ directions by a predetermined off-track amount.
However, the conventional off-track error recovery method has several disadvantages. First, the probability of reading data correctly based upon a predetermined off-track amount is low because a predetermined off-track amount can never be an optimum even though it has been obtained through experiments most likely to read data correctly from an off-track position.
If an arbitrary off-track amount is applied when another error exists in addition to an off-track error, such as a weak write error, the probability of the off-track error recovery method providing successful outcomes is even lower.
A weak Write error is an error where data cannot be written with sufficient write power even though a predetermined write current is normally applied to a head. For example, such a weak write error is more likely to occur on a data-storage medium at low temperatures because the coercive force of the data storage medium increases at low temperatures.